As a raw material of high-quality polyester fiber including polytrimethylen-terephthalate, the demand for 1,3-propanediol has increased in recent years. One of available methods for synthesizing 1,3-propanediol includes an acrolein hydration/hydrogenation method disclosed in Non-Patent Document 1. This method has been established as an industrial manufacturing method, where acrolein is obtained by synthesizing propylene as a petroleum raw material by air oxidation in the presence of a catalyst and such acrolein undergoes a hydration/hydrogenation reaction to manufacture 1,3-propanediol. However, with the sharp rise in oil price in recent years, the development of a synthesis method from a bio raw material has been demanded.
Although a method for chemically synthesizing 1,3-propanediol from a bio raw material has not been reported so far, a technique of synthesizing acrolein as a precursor is available, which is reported by Non-Patent Document 2, for example. Non-Patent Document 2 discloses a method of using glycerin as a bio raw material for a starting material to synthesize acrolein using supercritical water at 400° C. and 35 MPa. The technique has a feature in that protons from a very small amount of sulfuric acid added to supercritical water function as a promoter to accelerate a dehydration reaction of glycerin. According to this method, however, a mixture of tar and carbon particles is generated by heat decomposition as a by-product and such a by-product might block pipes and valves. To avoid such blockage, it is required in this reaction to use low concentration of raw materials so as to decrease the generation amount of the by-product. As a result, energy and cost used for the rise of temperature and pressure of water that are required per production volume become enormous, so that industrialization for mass production is difficult.
Patent Document 1 reports an exemplary supercritical reactor for removing solid particles such as salts. This technique is created based on the fact that while water has a high dielectric constant at ordinary temperatures and pressures and so has high solubility of salts, water in a supercritical condition tends to cause deposition of salts because of a decrease in dielectric constant. In this technique, in order to suppress the pipe blockage due to solid salts that are deposited beyond the solubility in supercritical water, a hydrocyclone is provided in a pipe for separation and capturing of solid matter. Conceivably, even such a technique has difficulty to be applied simply to a by-product as a target of the present invention. This is because the by-product contains high-viscosity and sticky tar and the adhesion of such a by-product to pipes and to a removal device for solid particles inhibits the operation of the apparatus.
The present inventors actually fabricated a prototype of a supercritical reactor illustrated in FIG. 1 of the same type as the apparatus described in Patent Document 1, and performed a synthesis experiment of acrolein under the conditions of glycerin concentration from 1.5% described in Non-Patent Document 2 to 15% ten times thereof. As a result, it was confirmed that while a very small amount of by-product was generated under the condition of low glycerin concentration of 1.5%, a large amount of by-product was generated in a reaction solution under the condition of high glycerin concentration of 15%, thus increasing the tendency of blockage at pipes, valves, and filters or at narrow sections such as a hydrocyclone. As the operation was continued, it was further confirmed that solid matter of the by-product adhered to valve bodies and valve seats, resulting in that the operation range of valve bodies was limited by wearing-out of the valve bodies and the valve seats, and it became difficult to control pressure precisely. It was further found that solid matter of the by-product accumulated at a lower part of a pipe, thus causing erosion of the pipe. Conceivably this resulted from the by-product particles having a large particle size and having adhesiveness because they were generated by agglomeration of carbon particles due to the adhesiveness of tar.    Non-Patent Document 1: Manufacturing, applications and economics of 1,3-PDO PTT, CMC Co. Ltd., Planet Business Dept., August 2000    Non-Patent Document 2: M. Watanabe, et al., Acrolein synthesis from glycerol in hot-compressed water, Bioresource Technology (Elsevier Ltd.) 98 (2007) pp. 1285-1290    Patent Document 1: JP Patent Application Publication No. 2000-279976 A